In general, a manual motor starter (so called MMS as abbreviated) serves as a switching device which has a function for protecting a motor by interrupting power supply to the motor upon generation of a fault current in a section for starting or stopping the motor, such as an electric shortage, a ground fault and an electric phase deficiency. The manual motor starter is also called a motor protection power-circuit breaker.
The manual motor starter is provided with a detecting section for detecting a fault current, a switching mechanism operated responding to a detection signal from the detecting section, a contact section interlocked with the operation of the switching mechanism to open or close a current path, and an arc extinguishing section for extinguishing an arc generated when contacts of the contact section are opened.
FIG. 1 is a schematic cross-sectional view of a conventional manual motor starter.
In FIG. 1, the reference numeral 1 designates a switching mechanism, 11 a crossbar, 21 a contact arm guide, 22 a movable contact arm, 22a movable contacts, 23a and 24a fixed contacts, and 25 an upper frame.
FIG. 2 is of perspective views illustrating operation of the switching mechanism for the conventional manual motor starter, wherein FIG. 2a is a perspective view illustrating the switching mechanism 1 in a state in which the manual motor starter is turned on and the contacts are closed, FIG. 2b is a perspective view taken at a different angle in FIG. 2a with a trip lever spring 6 omitted.
As shown in FIGS. 2a and 2b, the switching mechanism 1 comprises a latch 2, a trip lever 3, a latch holder 4 and a trip signal lever 5.
While the latch 2 is willing to rotate about its rotation shaft 2c due to a rotation moment 7 induced by a main spring (not shown), a leg portion 2a of the latch 2 is restrained by a stopper pin 4a of the latch holder 4. While the latch holder 4 is willing to rotate about its rotation shaft 4d due to the rotation moment 7 induced by the main spring which applies an elastic biasing force for rotating the latch 2 in a clockwise direction on a plane of the drawing, a head portion 4c of the latch holder 4 is restrained by a lock portion 3b of the trip lever 3. In order to prevent the trip lever 3 from rotating about its rotation shaft 3c in a counterclockwise direction, the trip lever spring 6 is connected between the trip lever 3 and the latch holder 4.
In this state, if the switching mechanism 1 of the manual motor starter implements trip operation, as driving force is transmitted to the trip lever 3 from the detection section, the trip lever 3 is rotated about the rotation shaft 3c in the counterclockwise direction on the plane of the drawing while overcoming an elastic biasing force of the trip lever spring 6. By this fact, the head portion 4c of the latch holder 4 is released from restraint by the lock portion 3b of the trip lever 3, and the latch holder 4 is rotated about its rotation shaft 4d in the counterclockwise direction. Further, as the leg portion 2a of the latch 2 is released from restraint by the stopper pin 4a of the latch holder 4, the latch 2 is rotated about its rotation shaft 2c in the clockwise direction due to the rotation moment 7 induced by the main spring. If the rotation moment 7 of the main spring vanishes, the latch 2 is stopped at a predetermined position by a stopper (not shown) to complete the trip operation, as shown in FIG. 3.
FIG. 3 are perspective views illustrating operation of the switching mechanism for the conventional manual motor starter, wherein FIG. 3a is a perspective view illustrating the switching mechanism in a state in which the manual motor starter is tripped and the contacts are opened, and FIG. 3b is a perspective view taken at a different angle in FIG. 3a with the trip lever spring 6 omitted.
If the trip operation of the switching mechanism 1 is implemented, as the latch 2 is rotated, a beak portion 2b of the latch 2 is also rotated about the latch rotation shaft 2c in the clockwise direction. At this time, as the beak portion 2b presses a projection 5a of the trip signal lever 5, the trip signal lever 5 is rotated about its rotation shaft 5e, and a leg portion 5b of the trip signal lever 5 is moved downward to perform a function of transmitting power to an accessory device for indicating a tripped state.
If the switching mechanism 1 is reset, the leg portion 2a of the latch 2 which is in a position shown in FIG. 3 is rotated about its rotation shaft 2c in the counterclockwise direction and is introduced between the stopper pin 4a and a power transmitting pin 4b of the latch holder 4. Thereafter, as the leg portion 2a of the latch 2 pushes the power transmitting pin 4b, the latch holder 4 is raised. Then, after the head portion 4c of the latch holder 4 passes while slightly lifting the trip lever 3, the head portion 4c of the latch holder 4 is moved back by an elastic biasing force of the trip lever spring 6 so that it is naturally restrained by the lock portion 3b of the trip lever 3 to define a turned-on state as shown in FIG. 2.
In the conventional manual motor starter, when the trip operation is implemented in a state in which the switching mechanism 1 is turned on and the contacts are closed, in the case that the head portion 4c of the latch holder 4 is incompletely released from restraint by the lock portion 3b of the trip lever 3, the latch 2 having a relatively large rotation moment 7 is barely released from restraint by the stopper pin 4a of the latch holder 4 and is rotated about its rotation shaft 2c to implement the trip operation for opening the contacts. However, the latch holder 4 cannot properly reach a trip completion position shown in FIG. 3 by interference with the trip lever 3 and other unexpected limitations, and still remains restrained by the trip lever 3 in an unstable state. In this state, if a user applies a force to a handle (not numbered) to reset the manual motor starter, the leg portion 2a of the latch 2 which is normally maintained in the trip position is rotated about the latch rotation shaft 2c toward the latch holder 4. At this time, since the latch holder 4 is maintained in an abnormal position in which it is not rotated in the counterclockwise direction, as can be readily seen from FIG. 4, the leg portion 2a of the latch 2 collides with the lower end of the latch holder 4. In this state, if the user further applies a force to the latch 2, the latch 2 or the latch holder 4 is likely to be broken, whereby the switching mechanism 1 may lose its functionality.